Integrated circuits are manufactured using photolithographic processes that direct light through a transparent photomask, also known as a reticle, to project a circuit image onto a silicon wafer. The quality of the integrated circuits thus produced may be adversely affected by any defects present in the mask. Because defects are not uncommon in the masks, these defects must be detected and repaired before using the mask in a production process.
Typically, automated mask inspection systems, such as those manufactured by KLA Instruments Corporation of San Jose, California, are used to detect defects. Such automated systems direct an illumination beam at the photomask and detect the intensity of the portion of the light beam transmitted through and reflected back from the photomask. The detected light intensity is then compared with expected light intensity, and any deviation is noted as a defect. The details of one such automated mask inspection system, as well as a review of others, can be found in U.S. Pat. No. 5,563,702 assigned to KLA Instruments Corporation.
Referring now to the drawing, wherein like reference numerals refer to like elements throughout, FIGS. 1 and 2 show cross-sectional illustrations of a type of phase-shifting photomask 10 undergoing an inspection for defects.
A phase-shifting photomask typically comprises a top surface 11, a bottom surface 13, opaque regions 12, and transmissive regions 14. Opaque regions 12 typically comprise an opaque coating 16, such as chrome, over top surface 11 of mask 10. Transmissive regions 14 may contain patterns of recessed features 18 in top surface 11 that, by changing the thickness of the mask 10 through which the light travels from bottom surface 13 to top surface 11, shift the phase of the transmitted light. Such recessed features 18 may vary in depth depending on the phase change desired, with a feature that produces a phase change of 180.degree. being twice the depth of a feature that produces a phase change of 90.degree., and so on. Non-etched areas 20 that are the full thickness of the mask 10 create no phase shift (0.degree.). Between each area of different mask thickness there is an edge 22.
Using a completed photomask 10, a light source is transmitted through the photomask 10 from the bottom surface 13, through reducing lenses (not shown), and onto a silicon wafer (not shown). It is desired to transfer an integrated circuit pattern onto the silicon wafer. A photosensitive layer applied to the wafer (not shown) reacts to the different phases of light projected onto it through the photomask 10, and thus creates the exposed pattern on the wafer corresponding to the features in the photomask 10.
To test the photomask 10, one type of automated tool may direct light 30 into mask 10 through bottom surface 13, as shown in FIG. 1, and record with a detector (not shown) the intensity of the transmitted light 31 exiting top surface 11. Similarly, another type of tool may reflect light 32 off the mask top surface 11, as shown in FIG. 2, and detect the intensity of reflected light 33 with a detector (not shown) on the same side of the mask 10 as the light source. Other tools may use a combination of reflected and transmitted light, and may shine light from one side of the mask 10 and detect both light transmitted to the other side and light reflected back from the same side of the mask 10.
Typically, a transmissive inspection tool directs an illumination beam of light 30 through a path from the bottom surface 13 to the top surface 11 of photomask 10. The tool detects the transmitted light 31 of the illumination beam of light 30 that is transmitted through photomask 10, and generates a signal 40 representative of the detected transmitted portion of light 30. The tool also has an expected transmission signal (not shown) corresponding to the transmitted portion expected to be detected and compares signal 40 to this expected transmission signal. Discrepancies greater than a specified threshold are recorded.
Typical reflective inspection tools work much the same as transmissive inspection tools, except that they detect reflected light 33 of an illumination beam of light 32 directed onto the top surface 11 of the photomask 10. A signal 42 representative of the detected reflected portion is compared to a stored expected reflection signal (not shown), and discrepancies greater than a specified threshold are recorded.
Tools using a simultaneous reflective and transmissive inspection technique may direct an illumination beam of light 30 through a path to the top surface 11 of the photomask 10 (as shown in FIG. 1), detect the transmitted light 31, and generate a signal 40 representative of the detected transmitted portion. In addition, however, such tools also detect a portion of illumination beam of light 30 reflected back from the top surface 11 of the photomask and the pattern of recessed features 18 on the photomask 10. A signal (not shown, but similar to signal 42) is generated representative of the reflected light 35 detected. Then, the signals representative of the reflective and transmissive portions of light 30 are compared to one another to create a comparison value (not shown). The comparison value is compared to a stored, expected comparison value (not shown), and a report (not shown) is generated when the comparison value does not correspond to the stored expected comparison value.
By the various inspection techniques, signals (such as 40 and 42) generated by the detection of the reflected or transmitted illumination beam may include disruptions (such as 44 and 44') corresponding to edges 22 and to certain defects 50. By comparing the generated signal 40 or 42 to the expected reflection or transmission signal, the inspection tool can distinguish expected, intentional disruptions, such as those caused by edge 22, from defect-caused disruptions, such as those caused by the edges of defect 50.
As the size of integrated circuitry continues to shrink to meet the industry demand for more performance from smaller devices, the wavelengths used by such automated inspection systems may be too large for the resolution needed. Thus, for instance, 248 nm phase-shifting photomasks may have defects not detectable by the currently available 488 nm and 365 nm inspection systems. The inspection tools are generally tailored for use with a specific, fixed wavelength, corresponding to the wavelength of the laser incorporated in the tool. Even as such inspection tools may be updated with shorter wavelength lasers, however, the ever-changing nature of the integrated circuit industry will likely continue to create at least short-term disconnects between available inspection devices and actual manufacturing needs.
Thus, as depicted in FIGS. 1 and 2, the signal disruptions 44 and 44' corresponding to defect 50 or to edge 22 may be relatively small with respect to the instrument sensitivity. To enhance the performance of such optically based inspection tools, typically an operator may adjust to concentrate on a smaller pixel size. Typical automatic inspection tools use a mathematical algorithm to analyze the light intensity data, so concentration on a smaller pixel size essentially enhances the ability of the algorithm to detect smaller defects by applying the algorithm over a smaller area. Even so, however, the sensitivity to defects depends on how much the defect alters the transmissivity or reflectivity of the photomask substrate material, and some defects may not cause enough of an alteration to be detectable at all.
Therefore, it is an object of the present invention to provide an improved method for detecting defects in photomasks that overcomes the limitations discussed above. Another object is to enable inspection devices to meet existing and anticipated actual manufacturing needs. A more specific object of the present invention is to alter the transmissivity or reflectance of defects existing in a photomask so that the corresponding disruption in light intensity detected by an instrument will be more significant.